1. Field of the Invention
The present invention relates to a pump for feeding a solvent, and more particularly to a pump for a liquid chromatograph device, which is accurate without any fluctuations in a flow rate in a wide flow range and a wide pressure range.
2. Description of the Related Art
In liquid chromatographic analysis, a sample to be analyzed is introduced into a separation column filled with filler particles having specific particle size and composition, and a single solvent or a mixture of a plurality of solvents is fed into the separation column by a pump. The sample introduced into the separation column is eluted for each component based on its chemical properties and composition, and the component is detected by a detector.
It is preferable that a solvent feed pump for liquid chromatograph can continuously feed a solvent at an accurate flow rate and in a stable state with few fluctuations in the flow rate under high pressure and in a wide flow range. Moreover, it is preferable that the solvent feed pump can mix a plurality of solvents and can perform a gradient operation of changing a mixture ratio of the solvents with time. Furthermore, the solvent feed pump is required to accurately and precisely control also a composition of the mixed solvents.
A variation in the flow rate of the solvents passing through the separation column or in the composition of the mixed solvents varies retention time of each component peak or peak height and area of each component on a chromatogram to be detected. The peak retention time is basic qualitative information, which takes a value unique to the component separated by the liquid chromatograph. The peak height and area indicate a concentration of the component separated by the liquid chromatograph. Therefore, the variation in the flow rate or in the composition of the mixed solvents causes uncertainty in peak identification. As a result, accuracy of quantitative measurement and reproducibility are impaired.
As a feed pump for continuous feeding, which is suitable for the liquid chromatograph, a dual piston pump is widely known, which includes a reciprocating piston in each of two pump heads connected to each other and uses one motor to drive a common camshaft designed to allow the piston to have predetermined phase difference and a predetermined stroke.
According to the technology described in Japanese Unexamined Patent Publication 55-128678 (1980), a cam designed to set the same amount of liquid flow at every point in a rotation cycle of a camshaft is used. Moreover, a pressure fluctuation is monitored by a pressure sensor installed inside a pump. In a case where the pressure is lowered at the start of discharge by a first piston, a rotational speed of a motor is increased by a certain multiplying factor to compress a solvent. Thus, a pulsating flow with diminished pulsing is realized by real-time control.
A control method using a pressure sensor described in Japanese Unexamined Patent Publication 55-128678 (1980) is a method realized on an assumption that a flow rate and the pressure are totally correlated with each other. For pressure measurement, there are pressure sensors having various principles. However, there are restrictions placed on the sensor, such as that the sensor has high fastness, reliability and stability even when used under high pressure for a long period of time, which are required of the liquid chromatograph, that the sensor has excellent corrosion resistance to various solvents such as an organic solvent, an acid solvent and an alkali solvent, that the sensor is small enough to be installed inside a device, that a detection part has a small solvent contact capacity, and that the sensor is excellent in responsiveness. Thus, as a typical pressure sensor, a semiconductor strain gauge pressure transducer is widely used, which converts a pressure into displacement through a diaphragm made of a stainless member or the like having excellent corrosion resistance, and takes out the displacement in the form of an electrical quantity by a bridge circuit.
However, even the pressure sensor described above requires response time of several hundred milliseconds under low pressure. Thus, the pressure sensor is insufficient for real-time control of a piston, which is required to operate at high speed. Therefore, Japanese Patent Publication No. 2604362 and Japanese Patent Publication No. 2564588 disclose pumps having a learning function for sequentially changing the number of revolutions and a phase of a motor for each period in a direction of reducing a pressure fluctuation by referring to a pressure profile of the previous period. However, such a pump has a drawback that the pump is vulnerable to such changes as a change in a flow rate and inclusion of bubbles.
The conventional technologies described above are all attempts to obtain a constant flow rate by constant control of the pressure fluctuation. In hydrodynamics, the Hagen-Poiseuille equation is known, which indicates that a flow rate and a pressure difference are proportional to each other, as a property of a flow inside a sufficiently developed straight circular tube.
However, as to an actual passage of liquid chromatograph, movements of a stirring member inside the passage and of a piston are added inside the passage to a configuration including various sizes and shapes of tubes, joints, blocks and the like. Accordingly, turbulent flows and vortexes are generated everywhere. Therefore, it is very difficult to accurately calculate correlation between the flow rate and the pressure.
Particularly, when the flow rate is increased, and turbulent flows are generated, the Hagen-Poiseuille equation by itself may not be sufficient to obtain a correlation between the pressure and flow rate. It is, therefore, virtually impossible to obtain a correlation between the pressure and the flow rate. Even if a composition and a temperature of a solvent are constant, a pressure difference inside the passage is sequentially changed by minute clogging and the like, and a detected pressure also fluctuates with time. Because of this, a minute leak from the joint and a seal part, which are provided inside the passage, cannot be eliminated. Consequently, it is very risky to discuss an absolute value of the flow rate based on an absolute value of the detected pressure.
In a case of a pump which has more than one pair of the foregoing dual piston pump units connected thereto and can perform a gradient operation of changing a mixture ratio of a plurality of solvents with time, accuracy and uniformity of a solvent composition to be synthesized become a key factor that determines performance of the liquid chromatograph. Accordingly, the pump units are required to have stricter flow rate accuracy and flow rate precision in a wide flow rate range.
Specifically, in such a pump which performs a high-pressure gradient operation, it is known that a minute flow rate error or flow rate fluctuation in each pump unit makes it difficult to feed the solvent while maintaining stable accuracy and uniformity of the solvent composition to be synthesized.
For example, Japanese Unexamined Patent Publication 8-15245 (1996) and 11-50967 (1999) disclose a gradient controller which predicts time when stable feeding would be disturbed by pressure interference between pumps, based on parameters determined by a set flow rate and a trial, and would perform feeding according to a pattern in a stable state without the interference during the predicted time.
However, in a case where flow rates in the respective pumps constantly vary, the time when the above disturbance in feeding (pressure fluctuation) would occur differs between the respective pumps. Therefore, it is difficult to accurately predict the period of time when the pressure interference would occur.
Therefore, a pump capable of feeding a solvent at a stable solvent mixture ratio is conceivable. Specifically, the pump has pressure sensors provided in a first pump chamber and in a passage after a second pump chamber. Moreover, the pump compresses the solvent by controlling pistons so as to set a pressure difference between the both pressure sensors to have a constant value, and corrects a pressure drop at a time when the inside of the first pump chamber is connected to the second pump chamber.
However, as described above, in the control by the pressure sensors, a problem with responsiveness of the pressure sensors in a low pressure region causes an uncontrollable error due to an incomplete correlation between the pressure and the flow rate in a high flow rate region. As a result, accuracy and precision of the solvent mixture ratio are impaired.
Moreover, the methods described above have a critical flaw as described below. A control error is caused by that zero points of both pressure sensors do not completely coincide with each other, and linear characteristics thereof do not, either. Since the control error is unavoidable, it is difficult to equally eliminate pressure fluctuations (flow rate fluctuations) over a wide flow rate range.